The principal use of a sedative-hypnotic drug is to produce drowsiness and to promote sleep. Since sedative-hypnotic drugs usually have the capacity of producing widespread depression of the CNS, these drugs are employed for various reasons, including as antiepileptic, muscle relaxants, antianxiety drugs, and even to produce amnesia or general anesthesia. Throughout the world, more prescriptions are written for sedative-hypnotic-antianxiety drugs than for any other class of drugs.
Barbiturates have enjoyed a long period of extensive use as sedative-hypnotic drugs. However, except for a few specialized uses, they have been largely replaced by the somewhat safer benzodiazepines.
The barbiturates reversibly depress the activity of all excitable tissues. Not all tissues are affected at the same dose or concentration. The CNS is the most sensitive to the action of barbiturates. When barbiturates are given in sedative or even hypnotic doses, there is very little effect on skeletal, cardiac, or smooth muscle. Even in anesthetic concentrations, peripheral effects are weak and do not create difficulties if the duration of anesthesia is not prolonged. However, if depression is extended, serious deficits in cardiovascular and other peripheral functions can occur.
Barbituric acid (2,4,6-trioxohexahydropyrimidine) and its analog thiobarbituric acid, lack central depressant activity, but the presence of alkyl or aryl groups at position-5 confers sedative-hypnotic and sometimes other activities. The general structural formula for the barbiturates and the structures of some of those available in the United States are shown in Table 1.
TABLE INAMES AND STRUCTURES OF SOME BARBITURATESAVAILABLE IN THE UNITED STATES. barbiturateR5aR5bAmobarbitalethylisopentylAprobarbitalallylisopropylBarbitalethylethylButabarbitalethylsec-butylButalbitalallylisobutylHexobarbital *methyl1-cyclohexen-1-ylMephobarbital *ethylphenylMetharbital *ethylethylMethohexital *allyl1-methyl-2-pentynylPentobarbitalethyl1-methylbutylPhenobarbitalethylphenylSecobarbitalallyl1-methylbutylTalbutalallylsec-butylThiamylal **allyl1-methylbutylThiopental **ethyl1-methylbutyl* R3 = H, except in hexobarbital, mephobarbital, metharbital, and methohexital, where it is replaced by CH3.** O, except in thiamylal and thiopental, where it is replaced by S.
The carbonyl group at position-2 has acidic properties because of its position between the two amido nitrogens, resulting in lactam-lactim tautomerization. The lactim (“enol”) form is favored in alkaline solutions, resulting in water-soluble salts. The lactam (“keto”) form does not dissolve readily in water, although it is quite soluble in non-polar solvents. Compounds in which the oxygen at C-2 is replaced by sulfur are called thiobarbiturates, which are more lipid-soluble than the corresponding barbiturates.
In general, structural changes that increase lipid solubility decrease duration of action, decrease latency to onset of activity, accelerate metabolic degradation, and often increase hypnotic potency. Introduction of polar groups such as ether, keto, hydroxyl, amino, or carboxyl groups into the alkyl side-chains decreases lipid-solubility and abolishes hypnotic activity. Methylation of the N-1 atom increases lipid-solubility and shortens the duration of action.
Convulsant seizures occur in various chronic central nervous system (CNS) disorders, particularly epilepsies. These seizures are generally correlated with abnormal and excessive EEG (electroencephalogram) discharges. A variety of drugs have been used for treatment of these seizures. Many of the older drugs are structurally related to phenobarbital, for example, the hydantoins, the deoxybarbiturates, the oxazolidinediones and the succinimides. More recently developed anticonvulsant compounds include the benzodiazepines, iminostilbenes, and valproic acid (Porter R. J., Meldrum, B. S. [1992] “Antiepileptic drugs” Basic & Clinical Pharmacology, Katzung B. G., Ed., Appleton & Lange, Norwalk, Conn., 5th Edition, pp. 331–349). Additional compounds, containing various types of chemical structures and having various pharmacological mechanisms of action are being developed because of their anticonvulsant activities (Trevor, A. J., Way, W. L. [1992] “Sedative-hypnotics” Basic & Clinical Pharmacology, Katzung, B. G., Ed., Appleton & Lange, Norwalk, Conn., 5th Edition, pp. 306–319).
The anticonvulsant drugs currently available in the United States have several shortcomings as therapeutic agents. About one of every three patients does not obtain significant relief from seizures and a number of side-effects accompany the therapeutic effects obtained.
The intravenous route of administration is usually reserved for the management of convulsive emergencies and for general anesthesia. Barbiturates are bound to plasma albumin to various extents. Lipid solubility is the primary determinant of binding. They also partition into fat in proportion to their lipid solubility. Highly lipid-soluble barbiturates such as thiopental, methohexital, thiamylal, thiohexital, and hexobarbital, undergo a rapid, flow-limited uptake into the most vascular areas of the brain. Maximal uptake occurs within 30 seconds after administration. There is then a redistribution into less vascularized areas of the brain and into other tissues. For such drugs there is no correlation between duration of action and elimination half-life. The highly vascular kidneys, liver, and heart equilibrate almost as fast as does the brain, so that maximum tissue concentration occurs within 1 minute after injection. The less lipid-soluble barbiturates equilibrate much more slowly because their uptake is limited by membrane permeability and not by blood flow. Cerebral uptake is slower and as long as 20 minutes may be required for sleep to occur after intravenous administration of barbital or phenobarbital. At steady-state, highest concentrations are achieved in fat which then acts as a slow-release reservoir of drug.
All barbiturates are filtered by the renal glomerulus in proportion to their free concentration in the blood. Barbiturates with a high lipid/water partition coefficient not only are highly protein bound and therefore are poorly filtered, but also are readily reabsorbed from the lumen of the tubule. The burden of elimination is thus put on the drug-metabolizing systems. When renal excretion is impaired, barbiturates that depend upon the kidney for elimination may cause severe CNS and cardiovascular depression. Small amounts of barbiturates are also secreted in milk. Metabolism occurs only in the liver for oxybarbiturates and to a small extent in the kidney for thiobarbiturates. The metabolism processes are oxidative in nature, leading to metabolites that are more polar and therefore more rapidly eliminated. The exception is the oxidative N-demethylation that leads to an active metabolite. The oxidative metabolism occurs mainly at carbon-5 where oxidation of radicals form alcohols, ketones, phenols, or carboxylic acids which may appear in the urine as such or as glucuronic acid conjugates. This process generally terminates biological activity.
Drug toxicity is an important consideration in the treatment of humans and animals. Toxic side effects resulting from the administration of drugs include a variety of conditions which range from low grade fever to death. Drug therapy is justified only when the benefits of the treatment protocol outweigh the potential risks associated with the treatment. The factors balanced by the practitioner include the qualitative and quantitative impact of the drug to be used as well as the resulting outcome if the drug is not provided to the individual. Other factors considered include the physical condition of the patient, the disease stage and its history of progression, and any known adverse effects associated with a drug.
Drug elimination is typically the result of metabolic activity upon the drug and the subsequent excretion of the drug from the body. Metabolic activity can take place within the vascular supply and/or within cellular compartments or organs. The liver is a principal site of drug metabolism. The metabolic process can be categorized into synthetic and nonsynthetic reactions. In nonsynthetic reactions, the drug is chemically altered by oxidation, reduction, hydrolysis, or any combination of the aforementioned processes. These processes are collectively referred to as Phase I reactions.
In Phase II reactions, also known as synthetic reactions or conjugations, the parent drug, or intermediate metabolites thereof, are combined with endogenous substrates to yield an addition or conjugation product. Metabolites formed in synthetic reactions are, typically, more polar and biologically inactive. As a result, these metabolites are more easily excreted via the kidneys (in urine) or the liver (in bile). Synthetic reactions include glucuronidation, amino acid conjugation, acetylation, sulfoconjugation, and methylation.
Absorption and redistribution are critical determinants of the time of onset and duration of anesthetic and hypnotic effects of ultrashort- and short-acting barbiturates. But it is the elimination rate that determines the time course of residual effects and accumulation of the drug during repetitive uses. Of all barbiturates currently used in the United States for hypnosis, only hexobarbital has a half-life of elimination that is sufficiently short (2.7–7 hours) for virtually complete elimination to occur within 24 hours. All other barbiturates will accumulate during repetitive administrations unless appropriate adjustments to dosage are made. Persistence of the drug in plasma furthermore favors the development of tolerance and abuse.
There is a need in the art for ultrashort acting hypnotic barbituates.